A nerve conduction study (NCS) is a diagnostic procedure whereby peripheral nerves are stimulated electrically and then bioelectrical potentials are recorded from the same nerve at a second location or from a muscle innervated by the activated nerve. These recorded bioelectrical potentials may, in many circumstances, provide a reliable indication of a particular medical condition.
Such nerve conduction studies often comprise both early potentials and late potentials. Early potentials reflect direct conduction from the site of stimulation to the site of recording. Late potentials represent conduction from the site of stimulation antidromically towards the spinal cord, reflection along the nerve path or within the spinal cord itself, and then conduction back down to the recording site.
The most common type of late potential recorded from a muscle innervated by the stimulated nerve is the F-wave. F-waves are highly variable waveforms that are caused by motor neuron back-firing and are generally recorded in all nerve conduction studies regardless of whether a pathology exists or not. F-wave study parameters generally include mean F-wave latency, minimum F-wave latency, maximum F-wave latency, chronodispersion, persistence, duration, F-wave amplitude, etc.
Due to the inherent variability in F-wave responses, these parameters must be established on the basis of multiple F-wave traces acquired after repetitive stimulation. Mean F-wave latency, the most robust F-wave parameter, generally requires the accumulation of at least ten individual F-wave latencies, or up to twenty stimuli, depending upon the persistence. Other parameters, such as minimum F-wave latency, may require the accumulation of even more individual F-wave latencies, as their physical attributes are typically less robust. Prior studies suggest that up to 30 F-wave latencies, or 100 stimuli, may be needed to reliably establish a number of the F-wave parameters. However, repetitive supramaximal stimulation can cause significant discomfort for some patients, thus limiting their willingness to accept high stimulus counts and, consequently, their acceptance for the procedure. More particularly, the reaction of a single muscle fiber to an electrical stimulus generally follows an all-or-nothing rule: each fiber of a muscle will either contract maximally or not at all. This means that a sufficient stimulus impulse (i.e., a “maximal stimulus”) is required to ensure that substantially all of the fibers of the stimulated muscle contract. However, some patients experience significant discomfort with such supramaximal stimulation, and are therefore reluctant to undergo repetitive supramaximal stimulation. This has the effect of limiting the utility of a repetitive supramaximal stimulation procedure.
Due to the aforementioned problems associated with a repetitive supramaximal stimulation procedure, efforts have been made to conduct nerve conduction testing using late potentials evoked with submaximal stimulation.
Currently, submaximal stimulation is manually determined and performed for F-wave analysis. Typically, a clinician gradually reduces the stimulation intensity level from its supramaximal level to a submaximal level by visually monitoring the direct muscle response changes occurring in response to the reduction in stimulation intensity. Subsequent F-wave analysis under submaximal stimulation is also manually performed by the clinician to determine the F-wave latency parameters, persistence level, and amplitude.
The current approach of manual submaximal stimulation and F-wave analysis has several significant deficiencies associated therewith. More particularly, one of the deficiencies of this approach is that the subjective determination of the submaximal stimulation level is time-consuming, and thus may not be performed because of time and resource limitations. In addition, the subjective determination of the submaximal stimulation level is operator dependent and, furthermore, inter-operator variations generally do not support the standardization of the submaximal F-wave analysis: these two factors may lead to wide differences in clinical results. Furthermore, the subjective extraction of submaximal F-wave features is generally time-consuming and may not be performed because of time and resource limitations. Also, the manual subjective processing of submaximal F-wave features limits, in a practical sense, the total number of waveforms which may be studied in a particular patient study and the precision of the features determined, thus making the diagnostic value of the F-wave studies less reliable.